This disclosure relates to a testing apparatus for orthopaedic specimens. In particular, this disclosure relates to a testing apparatus that is used to apply motions and forces to a test specimen(s) in a manner representative of what a prosthesis may encounter when implanted.
Various suppliers design and manufacture orthopaedic specimens in an effort to evaluate the suitability of a particular design for use such as a prosthesis, for example, a knee implant. Before these new designs are available for use, specimens must undergo rigorous testing under prescribed conditions. For example, ISO 14243 is a standard that sets forth criteria for evaluating the design and materials of knee implants, and particularly aids in evaluating the wear of test specimens. Imposed forces result in defined, discrete motions and the motions are coordinated with one another in a preselected environment (e.g., a force(s) applied in a particular pattern, for a desired time, at a desired velocity, and in a particular environment). The test is typically conducted for millions of cycles, for example, 5,000,000 to 10,000,000 cycles at 1 Hz. The test is extensive, carefully controlled, and test conditions are closely monitored, and preferably the testing apparatus can simultaneously test multiple, individual specimens under similar conditions.
For example, with reference to a knee implant and the noted ISO standard, a first defined motion (⊖y) caused by the moment (My) is generally referred to as flexion/extension and relates to rotation about one axis of an orthogonal coordinate system. The driving force or torque, to achieve this motion is applied to the specimen, and particularly the femoral component of the test specimen, while the other component is representative of the tibia.
A second defined motion (⊖z) caused by the moment (Mz) is rotation about one of the axes of the orthogonal coordinate system. This movement is representative of the movement of the tibia.
A third motion (X) caused by the force (Fx) is referred to as linear translation along one of the axes of the orthogonal coordinate system. In other words, this relates to forces that result in forward and backward motion imposed on the test specimen.
The fourth action relates to an axial compressive force (Fz) imposed on the test specimen. This axial force can rapidly increase in a short period of time. For example, a dynamic compressive load can rapidly increase by 1800 N in 0.03 seconds. Further, the dynamic, load must be able to exceed a force of 2600 N.
These motions and forces must be synchronized. At least three of the forces/motions/actions are periodic. Further, the testing system must maintain this synchronized action on multiple specimens over an extended period of time (e.g. testing of multiple specimens through millions of cycles takes a few months to complete).
Conventional, commercially available systems use multiple electrohydraulic actuators to achieve the various motions and load magnitudes required under the ISO standard. Unfortunately, these electrohydraulic actuators are relatively expensive. Further, the electrohydraulic actuators are not particularly effective in measuring smaller forces (e.g. on the order of less than 70 N) nor do the electrohydraulic actuators have good resolution. Also, use of the electrohydraulic actuators and associated controls require expensive sensors in an effort to achieve synchronization or phased movement as required under the ISO standard. As a result, the use of multiple electrohydraulic actuators, and the associated sensors and controls therefor, results in a test apparatus that is unduly expensive. These problems are magnified when the test apparatus is designed to simultaneously test multiple test specimens.
Accordingly, a need exists for an alternate test apparatus that is dependable, durable, accurate, easy-to-use, economical to manufacture and use, and can be easily adapted to multiple stations to permit simultaneous testing of multiple specimens.